1. Technical Field
The present invention relates to a Salen type ligand including three or more quaternary ammonium salts of nitrate anions, to a trivalent metal complex compound prepared from the ligand and a method of preparing the same, to a method of preparing polycarbonate by copolymerizing an epoxide compound and carbon dioxide using the complex compound as a catalyst, and to a method of separating and recovering the catalyst from the copolymer after copolymerization.
2. Description of the Related Art
Aliphatic polycarbonate is a polymer which is easily biodegradable, and is useful for example as a packaging material or a coating material. Preparation of polycarbonate from an epoxide compound and carbon dioxide is very environmentally friendly because phosgene which is poisonous is not used and carbon dioxide may be inexpensively obtained.
Many researchers have developed various types of catalysts in order to produce polycarbonate from an epoxide compound and carbon dioxide since 1960. The present inventors have recently disclosed a catalyst having high activity and high selectivity synthesized from a quaternary ammonium salt-containing Salen [H2Salen=N,N′-bis(3,5-dialkylsalicylidene)-1,2-ethylenediamine] type ligand [Bun-Yeoul Lee, Korean Patent No. 10-0853358 (issue date: 2008.08.13); Bun-Yeoul Lee, Sujith S, Eun-Kyung Noh, Jae-Ki Min, Korean Patent Application No. 10-2008-0015454 (filing date: 2008.02.20); Bun-Yeoul Lee, Sujith S, Eun-Kyung Noh, Jae-Ki Min, PCT/KR2008/002453 (filing date: 2008.04.30); Eun-Kyung Noh, Sung-Jae Na, Sujith S, Sang-Wook Kim, and Bun-Yeoul Lee* J. Am. Chem. Soc. 2007, 129, 8082-8083 (2007.07.04); Sujith S, Jae-Ki Min, Jong-Eon Seong, Sung-Jae Na, and Bun-Yeoul Lee, Angew. Chem. Int. Ed., 2008, 47, 7306-7309 (2008.09.08)]. The catalyst disclosed by the present inventors may be highly active and highly selective and enables the preparation of a copolymer having high molecular weight and polymerization to be carried out even at high temperature and thus may be applied to commercial processes. Furthermore, this catalyst is advantageous because a quaternary ammonium salt is contained in the ligand, and thus the catalyst may be easily separated from the copolymer after copolymerization of carbon dioxide and epoxide, and re-used.
Also, the present inventors have carefully examined a catalyst having higher activity and higher selectivity among the catalyst group of the above patent and thus have proved that such a catalyst has a peculiar structure in which a nitrogen atom of the Salen-ligand is not coordinated but only an oxygen atom is coordinated to a metal, which was not known to date (see Structure 1 below, Sung-Jae Na, Sujith S, Anish Cyriac, Bo-Eun Kim, Jina Yoo, Youn K. Kang, Su-Jung Han, Chongmok Lee, and Bun-Yeoul Lee* “Elucidation of the Structure of A Highly Active Catalytic System for CO2/Epoxide Copolymerization: A Salen-Cobaltate Complex of An Unusual Binding Mode” Inorg. Chem. 2009, 48, 10455-10465).

Furthermore, a method of easily synthesizing the ligand of the above compound has been developed (Min, J.; Seong, J. E.; Na, S. J.; Cyriac, A.; Lee, B. Y. Bull. Korean Chem. Soc. 2009, 30, 745-748). However, when considering the application of the highly active catalyst of Structure 1 to a commercial process, the following problems exist.
1) 2,4-dinitrophenolate or 2,4-dinitrophenol contained in the catalyst of Structure 1 is known to be explosive. Particularly the above material is known to be more explosive under dry conditions (K. R. Desai, B. G. Naik, Production of Organic Intermediates (Phamaceutical and Dyestuff), Sarup & Sons, New Delhi, 2005; p 5). The catalyst of Structure 1 should be prepared and stored under dry conditions. This catalyst is unsuitable for mass production and continuous use.
2) When carbon dioxide/epoxide copolymerization is carried out using the catalyst of Structure 1, a polymer chain grows from 2,4-dinitrophenolate or 2,4-dinitrophenol contained in the catalyst of Structure 1. Briefly, the 2,4-dinitrophenol group is mainly attached to the end of the polymer chain. Upon recovery of the catalyst, 2,4-dinitrophenolate and 2,4-dinitrophenol are not recovered. Attaching a 2,4-dinitrophenol group to an end of any polymer chain causes the price of resin to increase. Furthermore, the 2,4-dinitrophenolate anion is considerably yellow-colored, and thus the resin may be pale yellow-colored even after removal of the catalyst.
3) The preparation cost of the catalyst of Structure 1 is high. The synthesis method thereof is described below (Bull. Korean Chem. Soc. 2009, 30, 745-748). This method is inappropriate for mass production because of the following problems.

i) AgBF4 used in the first step is very expensive and thus mainly results in high catalyst preparation cost, and also mass production or purchasing thereof is not easy.
ii) The second step should be performed in an atmosphere without oxygen. Specifically, oxygen must be thoroughly removed from ethanol which is a solvent, but it is not easy to completely remove oxygen from ethanol.
iii) Sodium 2,4-dinitrophenolate used in the third step should be used in a dry state. In order to prepare dry sodium 2,4-dinitrophenolate, NaH and anhydrous THF solvent should be used. However, NaH is high-priced and has high ignitability and is thus difficult to mass handle. Furthermore, removing moisture from the THF solvent is a large burden in terms of mass production.
4) NaBF4 is used when recovering the catalyst of Structure 1, but is expensive, thus negating economic benefits.
Also analogous compounds having nitrate anions are disclosed in Chinese Patent (CN 100494248C (2009.06.03)), in which only compounds having a total of two quaternary ammonium salts to R5 and R6 and having four ammonium salts to R1, R2, R5 and R6 are described, which falls out of the range of a complex compound according to the present invention.